(1) Field of the Invention
The present invention relates to a magnetic recording disk and a ceramic substrate therefor. More particularly, the present invention relates to a ceramic substrate for a magnetic disk, which has no void defects on the surface and is excellent in the heat resistance, dimension stability and deformation resistance. Furthermore, the present invention relates to a process for the preparation of this ceramic substrate.
(2) Description of the Prior Art
A magnetic disk device plays an important role in the storing of information in a data processing system of a computer. In order to cope with a recent desire to increase the density and capacity in the magnetic recording, by utilizing a film-forming technique such as sputtering, the thickness of a magnetic recording medium is reduced, and the ratio of utilization of the area of a substrate is increased.
An aluminum alloy is used for the substrate on which a magnetic recording medium as mentioned above is formed, and an alumite layer having a thickness of 2 to 15 .mu.m, which is obtained by oxidizing the surface of the alloy, covers the surface of the substrate. Although the hardness of the surface of the substrate is increased by the presence of this alumite layer, since the thickness of the hard alumite layer is small and the thermal expansion coefficient of the aluminum alloy is different from that of alumite, distortion is readily caused as the substrate temperature arises. Namely, when a magnetic recording medium is formed on the substrate by sputtering, since the substrate is bombarded with sputtering particles or electrons, the temperature of the substrate is elevated by this bombardment energy. Furthermore, in case of a magnetic recording medium formed of .gamma.-Fe.sub.2 O.sub.3, a heat treatment is ordinarily carried out at a temperature higher than 300.degree. C. If the substrate undergoes such elevation of the temperature, distortion is readily caused in the aluminum substrate. Therefore, when a magnetic recording medium is formed on this aluminum substrate and is used for high-density magnetic recording, precise writing or reading is difficult.
In a magnetic disk device, a plurality of magnetic disks are arranged on the same rotation axis, and data processing such as writing or reading is carried out by rotating these disks at 1000 to 3000 rpm. If the disk substrate is formed of an aluminum alloy, the substrate per se is readily elongated by a centrifugal force. Also because of this elongation, it becomes difficult to perform precise writing or reading suitable for high-density magnetic recording. Accordingly, elimination of such writing or reading errors is desired.
An aluminum alloy substrate for a magnetic disk is ordinarily covered with an alumite layer as described above, but there are often present voids having a diameter of about 5 .mu.m at largest, and even an aluminum substrate having about ten voids is generally used. Accordingly, the conventional magnetic disk device for high-density recording is defective in that precise writing or reading is impossible because of the presence of such void defects. Therefore, development of a magnetic disk substrate having a reduced number of void defects is desired.
As another material for a disk substrate, plastics and glass have been examined. It is said that plastic materials are fatally defective in that the moisture permeability is high and glass materials are fatally defective in that glass is readily broken and the risk of damage by breaking is serious. Moreover, if a substrate formed of a plastic material is rotated at a high speed, since the Young's modulus of the plastic material is low, the substrate per se is readily elongated by a centrifugal force and the same problem as described above arises.